This application builds upon the disclosure of U.S. patent application Ser. No. 13/263,311, filed on Oct. 6, 2011, now U.S. Pat. No. 8,679,444, issued Mar. 25, 2014, which is a national phase entry of International Application Number PCT/US2010/029934, filed Apr. 5, 2010, and published in English as International Publication Number WO 2010/120581, which itself claims the benefit under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 61/170,199, filed Apr. 17, 2009, the disclosures of each of which are incorporated herein by reference.
Solid carbon has numerous commercial applications. These applications include longstanding uses such as uses of carbon black and carbon fibers as a filler material in tires, inks, etc., many uses for various forms of graphite (such as the use of pyrolytic graphite as heat shields) and innovative and emerging applications for buckminsterfullerene, carbon nanotubes and nanodiamonds. Conventional methods for the manufacture of various forms of solid carbon typically involve the pyrolysis of hydrocarbons in the presence of a suitable catalyst. The use of hydrocarbons as the carbon source is due to historically abundant availability and low cost of hydrocarbons. The use of carbon oxides as the carbon source in the production of solid carbon has largely been unexploited.
Carbon oxides, particularly carbon dioxide, are abundant gases that may be extracted from point source emissions such as the exhaust gases of hydrocarbon combustion or from some process off gases. Carbon dioxide may also be extracted from the air. Because point source emissions have much higher concentrations of carbon dioxide than air, they are often economical sources from which to harvest the carbon dioxide. However, the immediate availability of air may provide cost offsets by eliminating transportation costs through local manufacturing of the solid carbon products from carbon dioxide in air.
Carbon dioxide is increasingly available and inexpensive as a byproduct of power generation and chemical processes where an object may be to reduce or eliminate the emission of carbon dioxide into the atmosphere by capture and subsequent sequestration of the carbon dioxide (e.g., by injection into a geological formation). For example, the capture and sequestration of carbon dioxide is the basis for some “green” coal-fired power stations. In current practice, capture and sequestration of the carbon dioxide entails significant cost.
There are a limited number of ways that carbon, oxygen, and hydrogen can react to form solid carbon products and water. There is a spectrum of reactions involving these three elements wherein various equilibria have been identified that yield various allotropes and morphologies of solid carbon and mixtures thereof. Hydrocarbon pyrolysis involves equilibria between hydrogen and carbon that favors solid carbon production, typically with little or no oxygen present. The Boudouard reaction, also called the “carbon monoxide disproportionation reaction,” is the range of equilibria between carbon and oxygen that favors solid carbon production, typically with little or no hydrogen present. The Bosch reaction is within a region of equilibria where all of carbon, oxygen, and hydrogen are present under reaction conditions that also favor solid carbon production.
The relationship between the hydrocarbon pyrolysis, Boudouard, and Bosch reactions may be understood in terms of a C—H—O equilibrium diagram, as shown in FIG. 35. The C—H—O equilibrium diagram of FIG. 35 shows various known routes to solid carbon, including carbon nanotubes (“CNTs”). The hydrocarbon pyrolysis reactions are on the equilibrium line that connects H2 and C (i.e., the left edge of the triangle). The names on this line are of a few of the researchers who have published results validating CNT formation at various points on this line. The Boudouard, or carbon monoxide disproportionation reactions, are on the equilibrium line that connects O2 and C (i.e., the right edge of the triangle). The equilibrium lines for various temperatures that traverse the diagram show the approximate regions in which solid carbon will form. For each temperature, solid carbon may form in the regions above the associated equilibrium line, but will not generally form in the regions below the equilibrium line.
CNTs and other forms of nanocarbons are valuable because of their unique material properties, including strength, current-carrying capacity, and thermal and electrical conductivity. Current bulk use of CNTs includes use as an additive to resins in the manufacture of composites. Research and development on the applications of CNTs is very active with a wide variety of applications in use or under consideration. One obstacle to widespread use of CNTs has been the cost of manufacture. Thus, it would be desirable to provide reactors and methods that may be used to produce solid carbon materials, such as CNTs, in a more efficient manner.